The input-output model quantifies how inputs to economic sectors affect the overall economy, or the output, using linear equations. For example, the coal, steel and electricity industries all use similar raw materials for their processes. By defining the demand for these products (output), the input for raw materials can be quantified.
In economics, supply-and-demand graphs are determined by studying the behavior of sellers and buyers. As price for a product increases, sellers are motivated to sell more while buyers are motivated to buy less. You can graph this behavior by plotting on a graph in which the X-axis is quantity and the Y-axis is price of a product. Connecting the lines rarely yields a straight line. The least-squares method "smooths" the lines into straight lines so you can see a pattern.
Elasticity refers to how sensitive buyers or sellers are to changes in factors such as price and income. Elasticity is measured by the slope of a line, a concept learned in precalculus. For example, the slope of the demand curve is defined as the price elasticity of demand, or the sensitivity consumers are to a price change in a product.